Researchers say concerns have been raised about possible vision-related side effects associated with regular use of ED drugs. But their study showed no higher rates of vision loss or other eye complications among men who took Viagra or Cialis daily for six months.

Viagra and Cialis are part of a group of drugs known as selective phosphodiesterase type 5 (PDE5) inhibitors that treat erectile dysfunction by interfering with the action of PDE5 on the blood vessels in the penis.

Researchers say concerns have risen about possible long-term vision-related side effects of PDE5 inhibitors because they may also act on similar compounds in the retina, the part of the eye that receives and transmits images. Levitra is another PDE5 inhibitor drug used for ED, but was not used in this study.

Men taking these drugs have also reported cases of blurred vision, blue-tinged vision, and altered light perception.

In the study, published in the Archives of Ophthalmology, researchers randomly assigned 244 healthy men age 30 to 65, some with mild erectile dysfunction, to take a daily dose of 5 milligrams of Cialis, 50 milligrams of Viagra, or a placebo for six months.

The men had comprehensive vision exams at the beginning and at the end of the study, including electroretinography (ERG, a test used to detect diseases of the retina).

The results showed no significant differences between those who took the erectile dysfunction drugs or the placebo on ERG, vision function tests, pressure within the eyeball, or assessments of the anatomy of the eye.

"PDE5 inhibitors can exert direct effects on the retina, and such effects probably account for many of the visual side effects such as blue-tinged vision and light sensitivity that have been reported," writes study researcher William H. Cordell, MD of Lilly Research Laboratories, Eli Lilly and Company, which manufactures Cialis and supported the study. However, "our results indicate that there is no cumulative damage or effect of clinical significance for either 5 milligrams of tadalafil [Cialis] or 50 milligrams of sildenafil [Viagra] taken daily for six months."